As a wireless communication modulation/demodulation scheme, there is an OFDM (Orthogonal Frequency Division Multiplexing) modulation/demodulation scheme that is widely used in digital broadcasting or wireless LAN. This OFDM modulation/demodulation scheme has the characteristics that it has good frequency use efficiency through dense arrangement of sub-carriers by orthogonal frequencies, and is strong against frequency selective fading or reflected waves, and thus the use of the OFDM modulation/demodulation scheme in the next generation mobile communication has been examined.
In transmitting data in an OFDM modulation/demodulation scheme, it is necessary for a transmitting destination (receiving side) to detect a boundary of OFDM symbols in order to demodulate an OFDM-modulated signal. As a method of detecting a boundary of OFDM symbols, there is a method of detecting the boundary based on a correlation between a guard time and an OFDM-modulated signal. However, if delay waves exist due to reflection or the like, the detection accuracy is degraded, and thus in many cases, a transmission source (transmitting side) adds a signal for symbol synchronization to a transmitted audio signal.
In Patent Literature 1 described below, a data transmission system that makes data overlap an audio signal by applying the OFDM modulation/demodulation scheme to the audio signal is described. In the same manner as the wireless communication OFDM modulation/demodulation, the transmission system makes an OFDM-modulated signal and a signal for symbol synchronization overlap the audio signal. However, since the available frequency band is narrow, the system diffuses the signal for symbol synchronization and makes the diffused signal for symbol synchronization overlap the same frequency band as the audio signal.
Citation List
Patent Literature
Patent Literature 1: Japanese Patent Application Laid-open Gazette 2007-104598A